1. Field of the Invention
The present invention relates in general to a Metal-Oxide-Semiconductor (MOS) device, more particularly, to a MOS device having a localized punchthrough stopper adjacent to lightly-doped regions therein, which prevents punchthrough caused by the reduced channel length.
2. Description of the Prior Art
Punchthrough is a breakdown phenomenon caused by the widening of the drain depletion region when the reverse-bias voltage on the drain is increased. The electric field of the reverse-biased drain may penetrate into the source region and thereby reduce the energy barrier of the source-to-drain junction. Therefore, the shorter the channel length of the MOS device, the more likely the punchthrough phenomenon is to occur. It is evident that punchthrough is a severe problem in modern submicron devices.
At the present, there are at least three popular techniques for preventing punchthrough degradation in submicron MOS devices, which techniques are described below. A first approach for preventing the punchthrough phenomenon is to implant a punchthrough stopper accompanied by a conventional lightly-doped drain (LDD) implantation region. However, such an implantation may increase the concentration of the substrate near the source and drain junctions and thereby increase the junction capacitance. In addition, the impurities doped by the punchthrough stopper implantation may diffuse into the channel region, increasing the threshold voltage of this device.
A second approach is to utilize a large-angle-tilt implantation to form a punchthrough stopper near the boundary between the lightly-doped region and the device channel. Anti-punchthrough effect of this implantation is controlled by the doping energy and tilt angle. Those skilled in the art appreciate that it is difficult to control these factors precisely to achieve the intended effect. In addition, this anti-punchthrough implantation is implemented by variable doping angles, which may cause the impurity concentration in the source and drain regions to fluctuate undesirably.
A third popular approach is to use a technology called steep retrograde channel doping, which involves modifying the potential barrier from the source region to the drain region to decrease the possibility of punchthrough. However, it has been reported that this anti-punchthrough process may increase the body effect of this device, resulting in an increase in the threshold voltage.
Theoretically, an ideal punchthrough stopper should be located near the lightly-doped regions of MOS devices. Its impurities should be confined to a localized region and should not diffuse into other neighboring zones, such as the device channel. As can be seen, conventional anti-punchthrough techniques do not provide ideal punchthrough stoppers, especially in submicron MOS devices where the geometries are very small.